1. Field of the Invention
The present invention relates to a power conversion apparatus capable of increasing the stability of a multi-terminal direct current power transmission system in which a plurality of voltage power conversion apparatuses are connected to a direct current line, and also capable of continuously maintaining the operation of remaining power conversion apparatuses even if a terminal fault caused by a system fault or a fault of one of the power conversion apparatus occurs.
2. Description of the Prior Art
FIG.9 is a diagram showing a configuration of a conventional power conversion apparatus that has been disclosed in the Japanese Laid-Open Publication Number JP-A-1-238430. FIG.9 shows only one of a plurality of power conversion apparatuses connected to a direct current line. In FIG.9, the reference number 300 designates a power conversion apparatus to convert a Direct Current (DC) power to an alternating current (AC) power or the AC power to the DC power, 20 indicates an AC system, and 30 indicates a DC system. The reference number 301 designates a voltage type power conversion apparatus, 302 denotes an AC reactor or a transformer, and 303 indicates a capacitor. The reference number 304 designates an AC voltage detector for detecting the voltage of the AC system 20, and 305 indicates an AC current detector for detecting a current of the AC system 20. The reference number 306 designates a DC voltage detector for detecting the voltage of the DC system 30, and 307 indicates a power detection circuit to detect a power based on the voltage and the current detected by both the AC voltage detector 304 and the AC current detector 305. The reference number 308 designates a gate control circuit, 310 indicates a DC voltage control circuit, and 320 denotes a power control circuit. The reference numbers 311 and 321 designate subtracters, 312 and 322 denote compensators, and 330 indicates a selector.
Next, a description will be given of the operation of the conventional power conversion apparatus shown in FIG.9.
The power detection circuit 307 calculates an AC active power P=Vu*Iu+Vv*Iv+Vw*Iw based on each of three phase AC voltage values Vu, Vv, and Vw detected by the voltage detector 304 and each of three-phase AC values Iu, Iv, and Iw detected by the current detector 305. Then, the power detection circuit 307 outputs the calculated AC active power. Where, the reference character "*" designates a multiplication.
The AC active power P is approximately equal to the power that is converted by AC-DC conversion performed by the voltage type power convertor 301. The detected AC active power P is provided to the power control circuit 320. The subtractor 321 then calculates the deviation between the AC active power P and the power reference value Pref. The compensator 322 adjusts the power of the voltage type power convertor 301 according to the calculated the deviation of the power. That is, the compensator 322 increases the power of the voltage type power convertor 301 when the AC active power P detected by the power detector 307 is smaller in magnitude than that of the power reference value Pref. On the contrary, the compensator 322 decreases the power of the voltage type power convertor 301 when the AC active power P detected by the power detector 307 is larger in magnitude than that of the power reference value Pref. Thus, the compensator 322 controls the AC active power P such that P becomes equal in magnitude to the power reference value Pref.
On the other hand, the subtractor 311 calculates the deviation between the DC voltage Vd detected by the DC voltage detector 306 and the DC voltage reference value Vdref. Then, the compensator 312 adjusts the power of the voltage type power convertor 301 according to the calculated deviance of the voltage. Because the power of the voltage type power convertor 301 is adjusted bi-directionally, namely it is increased or decreased, the compensator 312 adjusts to increase the power from the AC to DC when DC voltage is decreased in order to charge the capacitor 303. On the contrary, the compensator 312 adjusts to decrease the power from the DC to AC when DC voltage is increased in order to discharge the capacitor 303. Thus, the compensator 312 adjusts Vd so that the value of the DC voltage Vd equals to the DC voltage reference value Vdref. Thus, both the compensators 312 and 322 adjust the power of the voltage type power convertor 301. In the conventional power conversion apparatus shown in FIG.9, the selector 330 selects the minimum value from the outputs of the compensators 312 and 322, and then one of the DC voltage control circuit 310 and the power control circuit 320 operates according to the selected value.
The gate circuit 308 controls to fire switching elements incorporated in the voltage type power convertor 301 according to the active power instruction Pac and the reactive power instruction Qac.
FIG.10 is a diagram showing a configuration of the voltage type power convertor 301 connected to AC power sources Vi and Vs through an AC reactor. In this case, when a voltage of the AC power source is Vs, a modulation index of the power convertor 301 is k, a reactance of the AC reactor is X, and a phase difference between the AC voltage of the voltage type power convertor 301 and the voltage of the AC power is .phi., it is well known that the active power and the reactive power may be obtained by the following equations: EQU P=k*sin .phi.*Vs.sup.2 /X (1) EQU Q=(k*cos .phi.-1)*Vs.sup.2 /X (2).
The gate circuit decides the timing to turn on and turn off switching elements in the voltage type power convertor 301 based on the modulation index k and the phase difference .phi. according to the equations (1) and (2), and then controls the switching elements in the voltage type power convertor 301. Thereby, the power of the voltage type convertor 301 is adjusted according to the outputs from the compensators 312 and 322 selected by the selector 330.
FIG. 11 is a diagram to explain the operation of the power conversion apparatus shown in FIG.9. In FIG. 11, the horizontal axis designates the AC active power P in which a positive direction from the left to the right indicates a power that is converted from AC to DC. The vertical axis indicates the DC voltage Vd of the voltage type power convertor 301.
The characteristic of the straight line showing a constant level of the voltage in a region shown by P&lt;Pref, as shown in FIG. 11, indicates that the output of the compensator 322 becomes greater according to the amplification of the compensator 322 and then reaches the limit value when the AC active power P detected by the power detection circuit 307 is smaller than the power reference value Pref.
Because the DC voltage Vd is designated near the DC voltage reference value Vdref, and the compensator 312 outputs the value within the output limit value, the selector 330 for selecting the minimum signal value selects the output from the compensator 312 to operate the DC voltage control 310, and the DC voltage of the voltage type power convertor 301 controls Vd so that it reaches the DC voltage reference value Vdref.
When the AC active power P becomes greater than the power reference value Pref, the compensator 322 outputs a small value and the compensator 312 outputs a large value because the DC voltage Vd is smaller than the DC voltage reference value Vdref, so that the selector 330 selects the output of the compensator 322 and then the power control circuit 320 operates. In FIG. 11, a straight line showing a constant value Pref in the region Vd&lt;Vdref indicates this characteristic. Accordingly, in the relationship between the power and the DC voltage in the conventional power conversion apparatus, the DC voltage control circuit 310 operates in one region and the power control circuit 320 operates in other region.
FIG. 12 is a diagram showing the connection configuration of conventional three power conversion apparatuses connected to the DC system. In FIG. 12, the reference numbers 300, 400, and 500 designate power conversion apparatuses, each having the same configuration as the conventional power conversion apparatus shown in FIG. 9. That is, each of the power conversion apparatuses 300, 400, and 500 has the configuration shown in FIG. 9.
In FIG. 12, the reference numbers 201, 211, and 221 indicate AC power sources, each of them is connected to each of the power conversion apparatuses 300, 400, and 500 through each of the AC systems 20, 21, and 22.
FIG. 13 is a diagram to explain the operation of the power conversion apparatuses having the configuration shown in FIG. 12. In FIG. 13, the solid line "a" indicates the power conversion apparatus 300, the dashed line "b" indicates the power conversion apparatus 400, and the dash-dotted line "c" denotes the power conversion apparatus 500.
In the case shown in FIG. 13, the DC voltage reference values Vdref1, Vdref2, and Vdref3 of the power conversion apparatuses 300, 400, and 500 are so set that they have the relationship Vdref1&gt;Vdref2&gt;Vdref3 and the power reference values Pref1, Pref2, and Pref3 are so set that they have the relationship Pref1&lt;Pref2&lt;Pref3.
As shown in FIG. 12, because three power conversion apparatuses 300, 400, and 500 are connected through the DC system 30 to each other, it must be required that they operate so that the sum of the powers of them becomes zero. For example, when the DC voltage is near the DC voltage reference value Vdref1, both the power conversion apparatuses 400 and 500 operate so that the power from AC to DC becomes small in order to decrease the magnitude of the voltage. Thereby, the DC voltage is decreased and the power control circuit 320 in the power conversion apparatus 300 operates.
Similarly, when the DC voltage is near the DC voltage reference value Vdref2, the power conversion apparatus 500 operates so that the power from AC to DC is decreased in order to decrease the DC voltage and the power conversion apparatus 400 operates under its control of the power control circuit.
Thus, when the three power conversion apparatuses having the configuration shown in FIG. 12 has the characteristic shown in FIG. 13, the power conversion apparatus 500 operates so that DC voltage has the DC voltage reference value Vdref3 and its power control circuit operates so that other power conversion apparatuses 300 and 400 have the power reference values Pref1 and Pref2. Accordingly, both the power conversion apparatuses 300 and 400 convert the AC power to the DC power, and the power conversion apparatus 500 converts the DC power, that is equal to the sum of the powers (Pref1+Pref2) of the power conversion apparatuses 300 and 400, to the AC power. The reference characters a', b', and c' designate the operation points of the power conversion apparatuses 300, 400, and 500, respectively.
We will consider the case in which the power conversion apparatuses having the configuration shown in FIG. 12 has the characteristic shown in FIG. 13. For example, because it is difficult to convert the DC power to the AC power when the operation of the power conversion apparatus 500 stops, the DC voltage is increased and the power conversion apparatus 400 operates so that the DC voltage becomes the DC voltage reference value Vref2. At this time, the reference characters a" and b" indicate the operation points of the power conversion apparatuses 300 and 400, respectively. The power control circuit 320 in the power conversion apparatus 300 is operating and the DC voltage control circuit in the power conversion apparatus 400 is operating so that the DC power that is equal to the power reference value Pref1 of the power conversion apparatus 300 is converted to the AC power.
As described above, when a plurality of conventional power conversion apparatuses are connected to the DC system 30, the DC voltage control circuit 110 in one of the power conversion apparatuses operates, and the power control circuits in the remaining power conversion apparatuses operate.
At this time, we will consider the case in which the power control circuit in the power conversion apparatus to convert the DC power to the AC power operates. When the voltage of the DC system 30 is decreased, a current flows so that the DC voltage is decreased in order to keep the power at a constant level. On the contrary, when the voltage of the DC system 30 is increased, the power control circuit 120 operates so that the DC voltage is further increased.
FIG. 14 is a circuit diagram to explain the above operation of the conventional power conversion apparatus. In FIG. 14, the configuration of each power conversion apparatus is the same as that of the power conversion apparatus shown in FIG. 9.
In FIG. 14, in the power conversion apparatus B in the power conversion apparatuses A, B, and C, the DC voltage control circuit of B operates as a voltage source, and the power control circuits in each of the power conversion apparatuses A and C operate as a current source.
When the AC active power P&lt;0 of the power conversion apparatus A is a constant value, the DC current becomes P/Vd. When the voltage Ed of the DC system 30 is decreased based on an accident, for example, the operation of the power conversion C stops, the power conversion apparatus B adjusts the power in order to keep the voltage at a constant level. However, the power conversion apparatus A operates so that the capacitor Ca is discharged (from DC to AC, because P&lt;0) corresponding to the decreasing of the DC voltage. Thereby, the power conversion apparatus B must provide a charging current for the capacitor Ca. But, when the length of the DC system is long, the power conversion apparatus B adjusts the power by using the voltage that is different from the voltage of the capacitor in the power conversion apparatus A. Accordingly, in some cases, deterioration of the control function of the DC voltage happens. Specifically, because it is often caused to increase a phase delay caused by resonance of an inductance component in the DC system and the capacitor of each power conversion apparatus, an underdamped voltage control characteristic is caused. When the distance of the DC system is long, the stability of the system comprising the power conversion apparatuses becomes jeopardized because the difference between the voltage of the capacitor requiring charging and the voltage adjusted by the DC voltage control circuit 110 becomes larger. In addition, when the capacitance of the capacitor C is smaller, the time length of discharging becomes short, the DC voltage control circuit must supply the charging current to the capacitor C. However, because the difference between the voltage of the capacitor requiring charging and the voltage adjusted by the DC voltage control circuit 110 increases, the control characteristic of the DC voltage control circuit 110 becomes undesirable and the degree of the stability of the system is decreased.
Thus, because the conventional power conversion apparatus has the configuration described above, one of a plurality of power conversion apparatuses controls the DC voltage and each of other power conversion apparatuses controls each power. Thereby, the power conversion apparatus to convert the DC power of the DC system 30 to the AC power of the AC system 20 controls so that the current further flows in order to decrease the DC voltage when the DC voltage of the DC system 30 is decreased, and, on the contrary, the DC voltage is further increased when the DC voltage of the DC system 30 is increased. As a result, the DC voltage control characteristic becomes unstable according to the increasing of the distance of the DC system 30 and the decreasing of the capacitance of the capacitor. There is therefore a drawback in the conventional power conversion apparatus that the deterioration of the DC voltage control circuit affects the ability of the AC system because it is difficult to adequately suppress a variation of the DC voltage caused by the resonance in the DC system 30.